Behavior of Suspended Sediment in the Changjiang Estuary in Response to Reduction in River Sediment Supply

Abstract

The Changjiang river sediment supply has decreased dramatically since 2003 when the Three Gorges Dam (TGD) began operation. Annually repeated cruises were conducted in the Changjiang estuary between 2002 and 2011 to examine the behavior of suspended sediment in response to the reduction in river sediment supply. Changes in the Changjiang estuary were not as clear as in the river and showed spatial and temporal variations. Generally, the suspended sediment concentration (SSC) in different channels showed different change characteristics. In the South Branch, furthest upstream, SSC showed no obvious changes for several years after the TGD started to operate in 2003 but then has been decreasing since 2009. In the South Channel, further downstream, SSC showed no obvious responses to the reduction of river sediment supply as of 2011. In the North Channel, SSC seems to have been decreasing slightly in recent years, though not as obviously as in the South Branch. Tidal resuspension contributed to temporal variability in SSC significantly and increased relatively at all stations. Vertical variations in particle size and texture in the South Branch in 2011 indicated very active exchange between the bed and the water column throughout a tidal cycle. The primary response of suspended sediment in the upper Changjiang estuary to the closing of the TGD in 2003 seems to be a lagged decrease in the fine fraction of suspended sediment. This conclusion is supported by multiple lines of evidence. Bulk settling velocities increased, riverine SSC and background SSC at slack tides decreased, and resuspension and deposition lags of SSC relative to current speed decreased. This response was strongest in the South Branch, moderate in the North Channel, and not yet apparent in the South Channel, indicating that the changes are still ongoing.